1. Field of the Invention
The present invention relates to a spread spectrum device, transmitter, receiver and communication system, and more particularly, to a multi-carrier spread spectrum device using a combination of cyclic-shift orthogonal keying (CSOK), transmitter, receiver and communication system, for use in a multi-carrier code division multiple access (MC-CDMA) transmission technique.
2. Description of Related Art
MC-CDMA, which is a multi-carrier transmission technique, combines advantages of both conventional Orthogonal Frequency Division Multiplexing (OFDM) and Code Division Multiple Access (CDMA) transmission technologies. In other words, the biggest advantage of MC-CDMA is that it can achieve frequency diversity and relatively higher bandwidth benefit at the same time. As in an OFDM system, MC-CDMA can allocate available frequency bands to several low transmission rate and orthogonal sub-carriers; and utilizes a cyclic prefix (CP) as guard interval to alleviate the inter-symbol interference (ISI) resulted from multi-path. MC-CDMA is different from Direct Sequence Code Division Multiple Access (DS-CDMA) that requires a high complexity rake receiver and interference-suppression technology to achieve a preferred frequency diversity effect. Utilizing an ordinary frequency-domain equalizing technology, MC-CDMA can still maintain a good bit error rate (BER) performance even under condition of multi-user coexistence. In other words, MC-CDMA can be considered as an OFDM system that applies a spread spectrum signal in the frequency-domain. When the spread spectrum factor equals 1, MC-CDMA becomes the OFDM system and it has no processing gain and the capacity of resisting channel fading. Conventionally, in a completely synchronous and ISI-free transmission environment, MC-CDMA uses the Walsh-Hadamard (WH) as the spread spectrum code to suppress the multi-user interference. However, the signal of MC-CDMA has a higher power average peak rate (PAPR), and the performance of MC-CDMA degrades greatly due to the inter-symbol interference (ISI). Therefore, when operating in a real channel environment, the conventional MC-CDMA tends to degrade with respect to the bandwidth benefit and power benefit.